1. Field of the Invention
The invention is directed generally to a heater apparatus, such as an auxiliary heater for a motor vehicle including one wall of a heat exchanger for separating a heated flammable gas or exhaust gas from a heat exchange medium, a conveyor mechanism for conveying the heat exchange medium along the wall, and a protective mechanism for protecting the heater from overheating. The invention further relates to a process for operating such a heater including the use of a hot-wire anemometer for measuring the hot air mass flow in the heater.
2. Description of the Related Art
Heater apparatus which are used as air heaters for heating hot air in a passenger compartment of a motor vehicle use fresh air and/or circulating air from the passenger compartment as the heat exchange medium. The hot air flow from the heat exchange medium absorbs the heat energy released by the heated exhaust gas to the wall of a heat exchanger and delivers the heat to the passenger compartment. When a passenger closes a flap or the exit openings of the passenger compartment heating system in order to limit the heat output from the heater, the hot air flow from the heater is reduced such that the heat exchange medium can no longer dissipate the heat energy of the flammable gas. Such a case is commonly referred to as xe2x80x9cdamming.xe2x80x9d In such a case, the temperature of the heat exchanger wall increases quickly, and results in the wall becoming leaky and can burn through. If this occurs, there is no longer separation between the exhaust gas and the heat exchange medium, and, in extreme cases, the exhaust can reach the passenger compartment.
In order to reliably prevent the dangerous mixing of the exhaust gas and the hot air flow, attempts have been, in the case of damming of the heater on the hot air outlet side, to promptly and reliably detect the danger of overheating of the wall of the heat exchanger, in order to then turn down or turn off the heater. Published German Patent Application DE 35 17 954 A1 discloses a generic auxiliary motor vehicle heater with a heat exchanger through which a heat exchange medium flows. In order to protect the heater from overheating, two sensors for sensing temperature are provided at the inlet and the outlet of the heat exchanger, the sensors being interconnected for monitoring of the operating state of the heater. The arrangement of two temperature sensors is, however, expensive and maintenance-intensive.
German Patent Publication DE 43 11 080 C1 discloses a motor vehicle heater which is independent of the engine and includes a flame monitor and one temperature sensor adjoining the wall of a heat exchanger for protecting the heat from overheating. Published German Patent Application DE 44 33 210 A1 discloses an auxiliary motor vehicle heater with a heat exchanger in which a temperature sensor is mounted between the combustion chamber and the exit opening of the heat exchange medium such that the sensor is used both as a mechanism protecting against overheating and as a flame monitor.
Temperature sensors for these applications generally have a resistance which can changed depending on temperature, for example, with a PTC characteristic. One disadvantage associated with this is that the shut-off of a PTC temperature sensor is comparatively slow and can only take place at a temperature above the full load temperature of the wall. For example, a full load temperature of 220xc2x0 C. is conventional, while a shutoff temperature is typically 250xc2x0 C. Therefore, under certain circumstances, the wall is heavily thermally loaded so that it must have a high wall thickness and other complex structural measures are necessary. Furthermore, to achieve the shutoff temperature, fuel is additionally consumed, with which the efficiency of the heater deteriorates.
Published German Patent Application DE 198 02 906 A1 discloses a fuel-operated air heater for motor vehicles with a burner and a heat exchanger by which hot air serves as the heat exchange medium and is conveyed by a fan, and flame monitor and an overheating sensor on the burner. The overheating sensor is an unencapsulated hot air temperature sensor, i.e., the sensor is located in the hot air flow without contact with the wall, especially in the area of the heat exchanger near the fan. Under certain circumstances, the measurement of the hot air temperature is too slow to reliably preclude overheating of the wall of the heat exchanger. This is due to the fact that when the hot air delivery is dammed, the temperature of the hot air rises very quickly and directly on the wall, but the temperature of the hot air rises more slowly farther away from the wall, for example at the site of the hot air temperature sensor.
Published German Patent Application DE 44 47 286 A1 discloses a motor vehicle heater with a burner (which is supplied by a fuel metering pump) and a combustion air fan. Depending upon the delivered amount of fuel, in order to achieve optimum combustion in the burner, a theoretical combustion air mass flow is determined and then compared to the actual combustion air mass flow. The speed of the combustion air fan is adjusted such that the actual value corresponds to the theoretical value. A combustion air mass flowmeter such as a hot-wire anemometer is placed in the combustion air channel in order to determine the actual combustion air mass flow. These combustion air mass flowmeters are known and are used in the control of combustion in the intake lines of internal combustion engines.
The object of the invention is to improve a heater of the initially-mentioned type such that the aforementioned disadvantages are surmounted, and especially, the danger of overheating the heater is recognized more reliably when the heat exchange medium delivery is dammed.
This object is achieved in accordance with the invention in that the protective mechanism of a heater of the initially-mentioned type is provided to determine during operation the mass flow of the heat exchange medium delivered by the conveyor mechanism.
The object is furthermore achieved with a process for controlling one such heater in which the protective mechanism, when the heater is activated, determines the mass flow of the heat exchange medium delivered by the conveyor mechanism, then compares the mass flow to the theoretical mass flow of the heat exchange medium stored in the protection mechanism, and at a difference between the current mass flow of the heat exchange medium and the theoretical mass flow of the heat exchange medium which indicates the danger of overheating the heat exchanger, limits the production of heated flammable gas in the heater.
The essence of the invention is that in order to detect the danger of overheating of the heat exchanger, the temperatures are not determined, as is the case in the related art which measures the temperature of the wall of the heat exchanger or the hot air as the heat exchange medium, but another physical quantity, specifically, the mass flow of the heat exchange medium, is determined. When the heat exchange medium inlet or the heat exchange medium outlet is dammed, the conveyor mechanism does not deliver the heat exchange medium in a sufficiently large amount through the heat exchanger, so that the mass flow of the heat exchange medium becomes zero or almost zero. This reduction in mass flow of the heat exchange medium is detected in accordance with the invention by the protective mechanism, and thus, the danger of overheating the heater is detected. The reduction of mass flow of the heat exchange medium occurs immediately upon damming so that the danger of overheating is detected without a skew. The heater in accordance with the invention can, therefore, be turned down or off especially quickly.
Another advantage of the invention is that the wall of the heat exchanger need not be heated beyond the full load temperature. The danger of damage to the wall or other components of the heater is, therefore, eliminated and additional fuel consumption is prevented. In addition, in the heater in accordance with the invention, the protection mechanism immediately and reliably detects failure of the conveyor for the heat exchange medium.
In one advantageous embodiment of the invention, the protective mechanism has a heat exchange medium mass flowmeter which is located in the hot air delivery line. A heat exchange medium mass flowmeter can be economically ordered as a standard component and is offered in various embodiments so that it can be easily incorporated electrically into an existing heater and its control. The heat exchange medium mass flowmeter is located advantageously in the conveyor line between the conveyor mechanism and the heat exchanger. In this section of the conveyor line, the heat exchange medium mass flowmeter can be installed easily since it does not require integration into the heat exchanger. The reaction time of the heat exchange medium mass flowmeter is especially short when it is located on the pressure side of the conveyor mechanism.
Another embodiment of the invention is that the heater includes a burner and the protective mechanism has a control device which is operationally coupled to the burner and the heat exchange medium mass flowmeter. The control device is provided to compare the acquired actual heat exchange medium mass flow to a theoretical heat exchange medium mass flow, and subsequently controls the burner depending upon the comparison, for example, by reducing output from burner or deactivating the burner. The proposed control is based solely on an actual-theoretical comparison, and, can therefore, be done by the control which is present in conventional heaters.
The function of the protective mechanism in accordance with the invention is expanded almost without additional cost by providing the control device with a nonvolatile storage in which, for the theoretical heat exchange medium mass flow, at least one tolerance range is filed which is assigned to one load state of the heater, especially a full load or at least a partial load. Preferably, the theoretical heat exchange medium mass flow values are stored for each load state of the heater. As long as the measured mass flow of the heat exchange medium and thus the delivery performance of the conveyor means is above the lower boundary of the tolerance range, sufficient flow of the heat exchange medium mass through the heat exchanger is ensured. As previously explained, the heat exchanger is protected against overheating. The upper boundary of the tolerance range ensures that the mass flow of the heat exchange medium is not unduly large. This could be the case when control of the conveyor mechanism or of the burner of the heater is defective or the load states of the burner and conveyor mechanism of the heat exchange medium are poorly tuned to one another. If during a certain unit of time relative to the load state of the burner, too much heat exchange medium mass is conveyed by the heat exchange medium, the heat exchange medium mass cannot be heated to the required extent and therefore does not yield the desired heat output in the passenger compartment.
In another embodiment of the invention which is especially well suited for heaters of motor vehicles, the heat exchange medium is air and the heat exchange medium mass flowmeter is a hot-wire anemometer. Mass flowmeters are already used in motor vehicles for controlling combustion in intake lines of internal combustion engines, and, therefore, are especially economical with respect to procurement, implementation and maintenance. Preferably, the hot-wire anemometer is coupled advantageously to a control device of a conventional auxiliary heater for motor vehicles by providing the hot-wire anemometer with a hot wire, such as a PTC hot wire. In operation of the heater, a constant electrical voltage is applied to the PTC hot wire and is cooled by the hot air mass flow so that its temperature, and thus, its resistance change. In this way, the intensity of a current flowing through the hot wire is a measure of the heat exchange medium mass flow.